These are the steps to turn on SSI's on Red Hat Linux w/ Apache, other distros should be very similar:

To turn on SSI's, Apache's httpd.conf needs to be edited.
Edit the file /etc/httpd/httpd.conf with your favorite editor, vi, pico, etc.
Inside this file, find your root directory for the site... should be something like, <Directory "/var/www/html">
Then inside this statement there should be a line that has all the options that are enabled for your root directory:

Options Indexes FollowSymLinks MultiViews +Includes

And what I'd do, is just tack on the +Includes on the end just like I've shown.

This will enable mod_include on the root directory and all subdirectories (that's what the plus sign indicates).

In addition, you may need to add these two lines, usually within the <IfModule mod_mime.c>, but check if they're already included.
AddType text/html .shtml
AddHandler server-parsed .shtml

Then just a simple 'sudo apachectl graceful' to restart apache, and includes should be turned on.